Best of 2005 Awards

Croton Dam Spillway Bridge

Award of Merit: Bridges

The bridge soars over Croton Gorge, spanning 200 ft. as part of the road that crosses the top of the Croton Dam, which at one time was the world's second-largest stone masonry structure and remains a landmark and engineering marvel.

But the spillway bridge, built in 1975 to replace an original structure erected in 1905, was slowly falling apart. Its design and the lack of anchor bolts or other restraints to upward movement contributed to severe displacement of the arch bases. That displacement, combined with the concentration of the bridge's load at the center of the arch, also led to overstressing of the arch ribs.

The New York City Department of Environmental Protection decided to replace the 1975 structure and ensure that the new bridge would have a much longer lifespan. Though located in Cortlandt, N.Y., in northern Westchester County, New York City owns the site, which is part of its vast municipal water system.

The project team - led by New York-based Hardesty & Hanover as structural engineer and Kiewit Constructors of Omaha, Neb., as general contractor - had to complete the job on a tight schedule. It also had to address community concerns over the poorly detailed aesthetics of the 1975 steel open-spandrel arch bridge.

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With a design schedule of only three months, the team quickly decided that rather than replicate the original 1905 span, it would incorporate modern materials and approaches into a structure sensitive to the historic context. The team chose spandrel columns that recreate the fundamental appearance of the original bridge while uniformly distributing deckloads to prevent displacement of the arch bases.

At a detail level, the railing, which was fabricated from steel castings and tubular steel, mirrors the spherical elements of the 1905 railing on a smaller scale. Such attention to aesthetics helped the resulting design to gain quick approval from the New York State Historic Preservation Office and the New York City Art Commission, winning commendations from both.

The team was able to expedite construction using an innovative erection scheme that leveraged the old bridge. The project was "actually a deconstruction-reconstruction in kind," said one juror impressed with the approach.

The team stabilized the old bridge by installing anchor beams in its arch bases and removing its deck. The team then used that structure to support the new bridge as construction proceeded.

The team installed new ribs to bear on the granite skewbacks on an alignment that stretched farther than the arches of the 1975 span - and that actually replicates the seating of the 1905 bridge. This allowed the team to leave the 1975 arches in place while installing the ribs.

The old arches, in turn, served as support for the installation of new arch segments. The new arches then served as a support for the team to disassemble the old arches. The new bridge still uses the original foundation, including the historic dam and masonry abutments.

The erection scheme also allowed crews to access the tricky worksite via the old structure. Temporary beams slung below the old arches supported a work platform and protective shield. These loads then transferred to the new arch ribs upon their installation, allowing the work platform and shield to remain in place throughout construction.

To ensure the structure's longevity in the constant mist rising from the spillway, High Steel Structures, a steel fabricator based in Lancaster, Pa., used three coating processes on 250 tons of steel. It put the interiors of tubes that were used as bracing elements through hot-dip galvanizing. It painted the insides of sealed welded boxes to avoid potential warping associated with the galvanizing. And it thermally sprayed the exposed exteriors and other visible components with zinc-aluminum metalizing.

The $4.6 million project finished on time in May.

Key Players

Owner: New York City Department of Environmental Protection

General Contractor: Kiewit Constructors

Engineer: Hardesty & Hanover

Owner's Representative: Zubatkin Associates

Steel Detailer: Tensor Engineering

Steel Fabricator: High Steel Structures

Tie Bags-Rock Bolts-Rock Anchors: CTC Drilling & Blasting

Expansion Joints: Kinedyne Corp.